1. Field of the Invention
The present invention relates to a phase change material heat exchanger wherein a phase change material comprising a salt selected for its relatively high latent heat is utilized as a heat storage medium. The heat exchanger of the invention comprises a substantially closed container into which the phase change material is placed. Then, using a system of conduits and discharge heads, a heat transfer fluid is passed through the phase change material so as to allow a heat energy exchange therebetween. This heat transfer takes place substantially at the heat of fusion of the phase change material so as to make optimum use of the material's latent heat. By virtue of the construction of the phase change material heat exchanger the use of homogenizing agents in combination with the phase change material is not necessary. In a preferred embodiment the phase change material heat exchanger is utilized as a heat storage facility in combination with a solar heat collector of state of the art construction. Solar heat is absorbed by the heat transfer fluid and stored by the phase change material placed within the heat exchanger. If solar heat is not currently available, heat previously stored could be used to raise the temperature of the heat transfer fluid.
2. Description of the Prior Art
Recent developments in the art of solar heating and cooling have created a great need for some means of efficiently storing the energy obtained from the sun for later use, such as at night or on cloudy days. A similar need has also been recognized with regard to the efficient operation of liquid-to-air heat pump systems. Such heat storage facilities are commonly referred to as heat sinks, and the prior art teaches numerous devices for the construction and operation of such heat storage facilities.
Perhaps the simplest of such devices consists essentially of a large holding tank into which the fluid which has been heated by the sun is collected for subsequent usage, such as a home's hot water supply. Other devices teach the storage of heat within rocks placed inside a container through which the heated fluid is allowed to flow. Most state of the art heat pumps utilize the atmosphere as a heat sink, either expelling waste heat to the air or extracting heat from the atmosphere, depending upon the mode of operation of the heat pump.
Each of these state of the art devices is relatively inefficient, and this inefficiency has become a primary concern because of the high energy associated with operating devices using these primary types of heat storage facilities. At least in partial solution of these problems, the current state of the art does teach what may be termed as secondary, or second generation, heat storage facilities.
These second generation heat storage facilities basically teach the use of a heat storage medium comprising a phase change material having a heat of fusion of more than 50 BTU per pound. By "phase change material" is meant a material which undergoes a physical change, such as from a crystal to a liquid or from an hydrated crystal to a dehydrated crystal, at a functional temperature. The bulk of the prior art teaches the use of salt hydrates as the phase change material, and it is the latent heat absorbed or expelled in accomplishing the phase change which is capable of being stored by the phase change material.
U.S. Pat. No. 2,677,664, to Telkes teaches the use of Glauber's salt (sodium sulfate decahydrate) as a suitable phase change material. However, as discussed in that patent, this material and other similar salt hydrates do not easily return to their normal hydrate form when giving up heat at their critical temperature without external agitation or stirring. That is to say, free water is often formed in the supersaturated solutions. This will not only tend to damage the container and heat storage system, but also deleteriously affects the system's efficiency. Telkes '664 teaches that this problem may be overcome by the addition of borax (sodium tetraborate decahydrate) into the salt hydrate solution. The same patent recognizes that mechanical means for external agitation of sealed containers of the heat storage medium would be possible, but is undesirable because of the costs and mechanics involved.
U.S. Pat. No. 3,986,969, also to Telkes teaches yet another solution to the problem of free water formation comprising not only the addition of borax as a nucleating agent, but also the addition of attapulgus clay (hydrous magnesium aluminum silicate) as an homogenizing agent. That patent teaches that the composition including the salt hydrate plus borax and clay maintains the salt hydrate in suspension during repeated heating and cooling cycles.
Still other examples of the use of phase change materials in heat storage and heat exchange devices are found in the following U.S. Patents: Nos.
Telkes 2,677,367 PA1 Telkes 2,808,494 PA1 Van Vechten 3,937,209 PA1 Boer 3,960,207 PA1 Switzgable 3,991,936 PA1 Chubb 3,997,001 PA1 Telkes 4,010,620
In addition to the crystallization/recrystallization problems discussed above, these patents also identify certain other problems found in state of the art heat exchanger devices using phase change material. For example, inasmuch as the phase change material usually consists essentially of a salt or a salt hydrate, direct contact of the phase change material with water is undesirable. Furthermore, because of the corrosive nature of the phase change materials utilized, relatively sophisticated and complex mechanical scraping, cleaning and agitating devices must often be utilized in the systems.
Accordingly, it is clear that there is a great need in the art for a heat exchanger construction wherein a phase change material may be efficiently utilized for the purpose of alternately storing and releasing heat energy. Such a heat exchanger device should be suitable for use in combination with existing heating and cooling systems and should be of relatively simple construction so as to provide long lasting, maintenancefree operation. For purposes of efficiency, it would also be desirable to utilize the phase change material in an unadulterated form without the necessity of incorporating nucleating and homogenizing agents.